1. Field of the Invention
This invention relates to a liquid jet recording head which performs recording by jetting a liquid to form flying liquid droplets.
2. Description of the Prior Art
Ink jet recording methods (liquid jet recording methods) are recently attracting attention because the noise they generate during recording is negligible, high speed recording is possible and also recording can be done on so-called plain paper without the need for special fixing treatments.
Among such methods, the liquid jet recording technique disclosed in, for example, Japanese Laid-open Patent Application No. 51837/1979, Deutsche Offenlegungsschrift (DOLS) 24843064 has a specific feature different from other liquid jet recording methods in that the driving force for discharging liquid droplets is obtained by permitting heat energy to act on a liquid.
More specifically, according to the recording method disclosed in the above patent specifications, liquid which has received the action of heat energy undergoes a change in state accompanied with an abrupt increase of volume, and through the acting force based on the change in state is discharged as liquid through the orifice at the tip end of the recording head section to be formed into flying liquid droplets, which liquid droplets are attached onto a material to be recorded, thereby effecting recording.
In particular, the liquid jet recording method disclosed in DOLS 2843064 is not only applicable very effectively for the so-called drop-on demand recording method, but also can easily be embodied into a recording head in which the recording head portion is made into a high density multi-orifice of the full line type, thus being capable of giving images of high resolution and high quality at high speed.
The recording head section of the device to be applied to the above-mentioned method has a liquid discharging portion having an orifice provided for discharging liquid and a liquid pathway, which is connected to the orifice and has a heat acting portion at which thermal energy acts on liquid for discharging liquid droplets, and an electro-thermal transducer as a means for generating thermal energy.
The electro-thermal transducer has a pair of electrodes and a heat-generating resistance layer which is connected to these electrodes and has a region for heat generation (heat-generating section) between these electrodes.
A typical example exhibiting the structure of such a liquid jet recording head is shown in FIG. 1A, FIG. 1B and FIG. 1C. FIG. 1A is the front view of a liquid jet recording head as seen from the orifice side, FIG. 1B is a partial sectional view of FIG. 1A when cut along the broken line X-Y and FIG. 1C is a plan view of the substrate.
The recording head 100 has a structure having orifices 104 and liquid discharging sections 105 formed by bonding a grooved plate 103 provided with a certain number of grooves of certain width and depth at a predetermined line density to a substrate 102 provided on its surface with an electro-thermal transducer 101 so as to cover the surface of the substrate 102. In the case of the recording head as shown in the drawing, it is shown as having a plural number of orifices 104. Of course, the present invention is not limited to such embodiments, but also a recording head with a single orifice is included in the category of the present invention.
The liquid discharging section 105 has an orifice 104 for discharging liquid at its terminal end and a heat acting portion 106 where thermal energy generated from an electro-thermal transducer 101 acts on liquid to generate a bubble and cause abrupt change in state through expansion and shrinkage of its volume.
The heat acting portion 106 is above the heat-generating section 107 of the electro-thermal transducer 101 and has a heat acting face 108 in contact with the liquid at the heat-generating section 107 as its bottom face.
The heat-generating section 107 is constituted of a lower layer 109, a heat-generating resistance layer 110 provided on the lower layer 109 and a first protective layer 111 provided on the heat-generating resistance layer 110. The heat-generating resistance layer 110 is provided on its surface with electrodes 113 and 114 for current flow through the layer 110. The electrode 113 is common to the heat-generating portions of the respective liquid discharging sections, while the electrode 114 is a selective electrode by selecting the heat generating portion of each liquid discharging section for heat generation and is provided along the liquid pathway of the liquid discharging section.
The first protective layer 111 has the function of separating the heat-generating resistance layer 110 from the liquid filling the liquid pathway of the liquid discharging section for protection of the heat-generating resistance layer 110 chemically or physically against the liquid employed at the heat-generating section 107, and also has the protective function for the heat generating resistance layer to prevent short-circuit through the liquid between the electrodes 113 and 114. The first protective layer 111 also serves to prevent electrical leaks between adjacent electrodes. In particular, prevention of electrical leakage between the respective electrodes or prevention of electric corrosion, which will occur by flow of electric current between the electrode under each liquid pathway and the liquid, which may happen to come into contact with each other for some cause, is important and for this purpose the first protective layer 111 having such a protective function is provided at least on the electrode existing under the liquid pathway.
Whereas, the upper layer, typically the first protective layer, is required to have characteristics which are different depending on the place at which it is to be provided. For example, at the heat-generating section 107, it is required to be excellent in (1) heat resistance, (2) liquid resistance, (3) liquid penetration preventing characteristic, (4) thermal conductivity, (5) antioxidant property, (6) insulating property and (7) breaking resistance, while in regions other than the heat-generating section 107, it must have sufficient liquid penetration prevention characteristics, liquid resistance and breaking resistance, although thermal conditions may be somewhat alleviated.
However, there is nowadays no material for constituting the upper layer which can satisfy all of the above characteristics (1) to (7) as desired, and under the present situation, some of the characteristics (1) to (7) are placed under alleviated requirements. That is to say, the choice of material in the heat-generating section is made with preference for characteristics (1), (4) and (5), while in other sections than the heat-generating section 107, for example, the electrode section, the choice of material is made with preference for characteristics (2), (3) and (7), thus forming the upper layers with the use of corresponding materials on the respective regional faces.
On the other hand, as different from these, in the case of a multi-orifice type liquid jet recording head, formation of respective layers and partial removal of the layers formed are conducted repeatedly on a substrate in the manufacturing step for the purpose of forming a number of minute electro-thermal transducers at the same time on the substrate. At the stage when the upper layer is formed, the surface on which the upper layer is to be formed is formed in minute ridged shape with a step wedge portion (stepped portion), and therefore the step coverage characteristic of the upper layer at this stepped portion becomes important. In short, if the step coverage characteristic at this stepped portion is poor, penetration of liquid will occur at that portion, whereby electric corrosion or breaking of electric insulation may be induced. Also, when the upper layer is susceptible to occurrence of failures at a fairly high probability during manufacturing, liquid will penetrate through the failures to markedly lower the life of the electro-thermal transducer.
For the reasons mentioned above, the upper layer is required to be good in step coverage characteristic at the stepped portion, low in probability of occurrence of pinholes in the layer formed or the pinholes, if any, are so few as to be negligible.
Accordingly, it has been practiced in the prior art, so as satisfy these requirements, to form the upper layer of a laminate of a first protective layer consisting of an inorganic insulating material and a second protective layer consisting of an organic material, or further to constitute a first protective layer of a double-layer structure and a lower layer of an inorganic insulating material, or to constitute an upper layer of an inorganic material which is tenacious, relatively excellent in mechanical strength and can be closely contacted and adhered with the first protective layer and the second protective layer, for example, a metal. Alternatively, a third protective layer constituted of an inorganic material such as a metal was arranged further on the second protective layer.
Although the second protective layer constituted of an organic material is excellent in coating characteristic, it is inferior in heat resistance, and therefore it is formed in a pattern as shown in FIG. 1C. However, in the case of such a constitution, the partition wall of an organic material is formed on the orifice surface formed by cutting, and said partition wall receives force during cutting to lower the mechanical strength. At the portion where mechanical strength is lowered, a part of the flying liquid droplets transmitted from the orifice surface will be penetrated to lower adhesion of the second protective layer to cause peel-off of the layer. For this reason, electrical leakage into the liquid in the liquid pathway is increased, whereby there ensues the problem that stable formation of flying liquid droplets is inhibited.
Further, since the second protective layer is formed at the portion excluding the heat-acting surface due to the characteristics of the material as mentioned above, a high step difference will result near the heat-acting surface. Higher density liquid jet recording heads are susceptible to formation of step difference near the heat-acting surface. On the other hand, in the vicinity of the heat-acting surface, foaming and condensation are repeated at a frequency of some thousand times per second, and the pressure change thereby formed will frequently destroy the second protective layer formed near the heat-acting surface.